We derive effective Hubbard-type Hamiltonians of κ-(ET)2X, using an {\em ab initio} downfolding technique, for the first time for organic conductors. They contain dispersions of the highest occupied Wannier-type molecular orbitals with the nearest neighbor transfer t∼0.067 eV for a metal X=Cu(NCS)2 and 0.055 eV for a Mott insulator X=Cu2(CN)3, as well as screened Coulomb interactions. It shows unexpected differences from the conventional extended H\"uckel results, especially much stronger onsite interaction U∼0.8 eV (U/t∼12-15) than the H\"uckel estimates (U/t∼7-8) as well as an appreciable longer-ranged interaction. Reexamination on physics of this family of materials is required from this realistic basis.
@article{arxiv.0903.5409,
title = {Ab initio Derivation of Low-Energy Model for $\kappa$-ET Type Organic Conductors},
author = {Kazuma Nakamura and Yoshihide Yoshimoto and Taichi Kosugi and Ryotaro Arita and Masatoshi Imada},
journal= {arXiv preprint arXiv:0903.5409},
year = {2010}
}